1. Field of the Invention
The present invention relates to the use as fertilizers of certain prussiates (prussides or pentacyanoferrates), some of which are known.
2. Discussion of the Prior Art
In order to achieve optimum growth conditions for a plant, in addition to the macronutrients, the micronutrients must also always be present in a substrate in a sufficient amount, and in particular in such a form that they can be taken up by the plant. It is therefore frequently necessary either to compensate a deficiency of micronutrients available to plants in substrates which are used for growing plants by adding appropriate micronutrient fertilizers, or to remedy the deficiency by converting the sparingly soluble micronutrient compounds contained in the substrate into more readily soluble substances. Fertilization with trace elements can thus be carried out in a variety of ways.
For example, it has been known for a long time that readily soluble salts of trace element cations with nonphytotoxic acids can be employed for supplying plants with the relevant micronutrients. Using conventional fertilizers of this type, it is indeed possible to achieve an effective micronutrient supply in weakly acid substrates or in substrates having a neutral reaction; however, their use in soils having a weekly basic reaction suffers from considerable disadvantages. Thus, the majority of polyvalent metal ions, in particular iron ions, cannot be taken up by the plants because, in a weakly alkaline substrate, these ions separate out in the form of sparingly soluble hydroxides and therefore do not contribute to plant nutrition.
Furthermore, it has been disclosed in "Der Vegetations-versuch" ("Vegetation Experiments") in "Methodenbuch" ("Book of Methods"), volume VIII, Neuman Verlag, Radebeul, Berlin, 1951, 180-194; Plant Physiology 26, 411 (1951); Soil Science 80, 101-108 (1955) and "Organic Sequestering Agents", John Wiley and Sons, Inc. New York, 1959, 455-469, that the plants can be supplied with the necessary micronutrient cations in the form of chelate complexes of citric acid, gluconic acid, nitrilotriacetic acid, ethylenediaminetetraacetic acid and ethylenediamine-N,N'-di-(o-hydroxyphenyl)-acetic acid. With the aid of complexes of this type it is possible to achieve a supply of micronutrients to the plants not only in weakly acid or neutral soils but also, to a certain degree, in weakly alkaline soils because, as a result of the relatively high stability of these complexes, an undesired precipitation of the micronutrient cations in the neutral or weakly basic medium is largely prevented. Nevertheless, the use of chelate complexes for the indicated purpose suffers from some disadvantages. Thus, the duration of action of chelate complexes of citric acid or gluconic acid is only relatively short, since these naturally occurring acids can be degraded fairly rapidly by soil micro-organisms.
The chelate complexes of the synthetic aminopolycarboxylic acids, with the exception of the iron complex of ethylenediamine-N,N'-di-(o-hydroxyphenyl)-acetic acid, which is important for combating chlorosis, can be employed only with limitations in strongly alkaline soils, because the stability of the complexes does not always suffice to avoid the micronutrient cations being immobilised in the form of sparingly soluble hydroxides or oxides. A further disadvantage is that the aminopolycarboxylic acids form very stable, highly toxic and at the same time water-soluble chelate complexes with the heavy metal ions of cadmium, lead and mercury, which can be contained in the soil in the form of almost insoluble compounds. Since these heavy metal ion complexes can, because of their good solubility, pass into the soil water, the use of aminopolycarboxylic acids is not acceptable for toxicological reasons. It is true that the iron complex of ethylenediamine-N,N'-di-(o-hydroxyphenyl)-acetic acid is, as already mentioned, of practical importance in combating chlorosis. However, it is a disadvantage that this substance can be prepared only with relative difficulty and is furthermore not light-stable.